For many diseases, a more accurate diagnosis can be made if an image of the body tissue affected by the disease can be observed. However, many body tissues are not readily observable. Recently, ultrasonic imaging has been widely used for diagnosing diseases in a body cavity, such as in the vascular system, gastrointestinal tract, and the like. This involves inserting an ultrasonic probe into the targeted body area with a catheter. The ultrasonic probe transmits an acoustic pulse into the body and detects the reflections of the pulse at tissue boundaries due to differences in acoustic impedance. The differing times taken for the transducer to receive the reflected pulse correspond to variations in the distance of the tissue boundaries from the ultrasonic source. By stepping, or sweeping, the ultrasonic probe through a selected angle, a two dimensional ultrasound image corresponding to a map of the acoustic impedance boundaries can be obtained. The intensity and position of the reflections from these boundaries will provide information on the condition of the body tissue being imaged.
Generally, there are two types of ultrasonic probes for diagnostic ultrasonic imaging. The first employs a synthetic aperture technique. For example, U.S. Pat. No. 4,917,097 (Proudian et al.) and U.S. Pat. No. 5,186,177 (O Donnell et al.) teach how an ultrasonic beam is steered electronically from a transducer using the method of synthetic aperture. Generally, this involves the sequential excitation of selected elements in an array of transducer elements. The second scans by mechanical rotation of a means to direct acoustic pulses. The mechanically rotated type includes a few subclasses. In the first subclass, either the distal (remote from the operator) transducer or a mirror is rotated from the proximal end of the catheter by an extended drive shaft with a proximal motor (U.S. Pat. No. 4,794,931 (Yock) and U.S. Pat. No. 5,000,185 (Yock)). In the second subclass, the rotation is confined to the distal end, where either a miniature motor (U.S. Pat. No. 5,240,003 (Lancee et al.) and U.S. Pat. No. 5,176,141 (Bom et al.)) or a fluid driven turbine is used to rotate the transducer or the mirror (U.S. Pat. No. 5,271,402 (Yeung and Dias)). In a third subclass, a stationary proximal transducer is acoustically coupled to a rotating acoustic waveguide that conducts the sound to the distal end (e.g., U.S. Pat. No. 5,284,148 (Dias and Melton). In another subclass, e.g., U.S. Pat. No. 5,509,418 (Lure et al.), a turbine is rotated by an coustic signal generated outside the vascular vessel to direct another ultrasonic signal in a rotating fashion. In yet another subclass, e.g., U.S. Pat. No. 5,507,294 (Lum et al.), an external driving member rotates a tube to rotate a reflecting element at the tip of the tube to reflect ultrasound.
Currently, the most widely used type of intracavity ultrasonic probe is the mechanically rotated system with a transducer having a single planar element placed at the distal end of the catheter. A reason for this preference is the superior image quality as compared with current synthetic aperture systems. However, the mechanically rotating ultrasonic probes have some shortcomings. For an ultrasonic probe with a drive motor proximal to the operator, i.e., remote from the transducer, a drive cable encircled by a sheath is generally needed to transfer mechanical energy to the tip of the catheter containing the transducer. A long cable may not transfer energy uniformly to the catheter tip to rotate the transducer or reflector uniformly. Furthermore, the probe is liable to fail over time because of the cable's rapid and repetitive rotation within the sheath. On the other hand, when a drive motor is located near the tip of the catheter, the motor must be small. Such fragile motors are electrically and mechanically complex, making them very expensive. With mechanical parts, e.g., ball beatings, etc., that undergo rigorous motion, the motor is liable to fail. What is needed is an ultrasonic probe with a structurally simple actuator at the tip of the catheter for moving a transducer to scan tissues.